1. Field of the Invention
The present invention relates, generally to a cooling circuit for a dual clutch transmission and, more specifically, to a hydraulic circuit used to controlling the flow of cooling fluid provided to each of the two clutches of a dual clutch transmission.
2. Description of the Related Art
Generally speaking, land vehicles require a powertrain consisting of three basic components. These components include a power plant (such as an internal combustion engine), a power transmission, and wheels. The power transmission component is typically referred to simply as the “transmission.” Engine torque and speed are converted in the transmission in accordance with the tractive-power demand of the vehicle. Presently, there are two typical transmissions widely available for use in conventional motor vehicles. The first, and oldest type is the manually operated transmission. These transmissions include a foot-operated start-up or launch clutch that engages and disengages the driveline with the power plant and a gearshift lever to selectively change the gear ratios within the transmission. When driving a vehicle having a manual transmission, the driver must coordinate the operation of the clutch pedal, the gearshift lever and the accelerator pedal to achieve a smooth and efficient shift from one gear to the next. The structure of a manual transmission is simple and robust and provides good fuel economy by having a direct power connection from the engine to the final drive wheels of the vehicle. Additionally, since the operator is given complete control over the timing of the shifts, the operator is able to dynamically adjust the shifting process so that the vehicle can be driven most efficiently. One disadvantage of the manual transmission is that there is an interruption in the drive connection during gear shifting. This results in losses in efficiency. In addition, there is a great deal of physical interaction required on the part of the operator to shift gears in a vehicle that employs a manual transmission.
The second, and newer choice for the transmission of power in a conventional motor vehicle is an automatic transmission. Automatic transmissions offer ease of operation. The driver of a vehicle having an automatic transmission is not required to use both hands, one for the steering wheel and one for the gearshift, and both feet, one for the clutch and one for the accelerator and brake pedal in order to safely operate the vehicle. In addition, an automatic transmission provides greater convenience in stop and go situations, because the driver is not concerned about continuously shifting gears to adjust to the ever-changing speed of traffic. Although conventional automatic transmissions avoid an interruption in the drive connection during gear shifting, they suffer from the disadvantage of reduced efficiency because of the need for hydrokinetic devices, such as torque converters, interposed between the output of the engine and the input of the transmission for transferring kinetic energy therebetween. In addition, automatic transmissions are typically more mechanically complex and therefore more expensive than manual transmissions.
For example, torque converters typically include impeller assemblies that are operatively connected for rotation with the torque input from an internal combustion engine, a turbine assembly that is fluidly connected in driven relationship with the impeller assembly and a stator or reactor assembly. These assemblies together form a substantially toroidal flow passage for kinetic fluid in the torque converter. Each assembly includes a plurality of blades or vanes that act to convert mechanical energy to hydrokinetic energy and back to mechanical energy. The stator assembly of a conventional torque converter is locked against rotation in one direction but is free to spin about an axis in the direction of rotation of the impeller assembly and turbine assembly. When the stator assembly is locked against rotation, the torque is multiplied by the torque converter. During torque multiplication, the output torque is greater than the input torque for the torque converter. However, when there is no torque multiplication, the torque converter becomes a fluid coupling. Fluid couplings have inherent slip. Torque converter slip exists when the speed ratio is less than 1.0 (RPM input>than RPM output of the torque converter). The inherent slip reduces the efficiency of the torque converter.
While torque converters provide a smooth coupling between the engine and the transmission, the slippage of the torque converter results in a parasitic loss, thereby decreasing the efficiency of the entire powertrain. Further, the torque converter itself requires pressurized hydraulic fluid in addition to any pressurized fluid requirements for the actuation of the gear shifting operations. This means that an automatic transmission must have a large capacity pump to provide the necessary hydraulic pressure for both converter engagement and shift changes. The power required to drive the pump and pressurize the fluid introduces additional parasitic losses of efficiency in the automatic transmission.
In an ongoing attempt to provide a vehicle transmission that has the advantages of both types of transmissions with fewer of the drawbacks, combinations of the traditional “manual” and “automatic” transmissions have evolved. Most recently, “automated” variants of conventional manual transmissions have been developed which shift automatically without any input from the vehicle operator. Such automated manual transmissions typically include a plurality of power-operated actuators that are controlled by a transmission controller or some type of electronic control unit (ECU) to automatically shift synchronized clutches that control the engagement of meshed gear wheels traditionally found in manual transmissions. The design variants have included either electrically or hydraulically powered actuators to affect the gear changes. However, even with the inherent improvements of these newer automated transmissions, they still have the disadvantage of a power interruption in the drive connection between the input shaft and the output shaft during sequential gear shifting. Power interrupted shifting results in a harsh shift feel that is generally considered to be unacceptable when compared to smooth shift feel associated with most conventional automatic transmissions.
To overcome this problem, other automated manual type transmissions have been developed that can be power-shifted to permit gearshifts to be made under load. Examples of such power-shifted automated manual transmissions are shown in U.S. Pat. No. 5,711,409 issued on Jan. 27, 1998 to Murata for a Twin-Clutch Type Transmission, and U.S. Pat. No. 5,966,989 issued on Apr. 4, 2000 to Reed, Jr. et al for an Electro-mechanical Automatic Transmission having Dual Input Shafts. These particular types of automated manual transmissions have two clutches and are generally referred to simply as dual, or twin, clutch transmissions. The dual clutch structure is most often coaxially and co-centrically configured so as to derive power input from a single engine flywheel arrangement. However, some designs have a dual clutch assembly that is coaxial but with the clutches located on opposite sides of the transmissions body and having different input sources. Other designs are known in which the two clutches are non-coaxial with differing input sources. Regardless, the layout is the equivalent of having two transmissions in one housing, namely one power transmission assembly on each of two input shafts concomitantly driving one output shaft. Each transmission can be shifted and clutched independently. In this manner, uninterrupted power upshifting and downshifting between gears, along with the high mechanical efficiency of a manual transmission is available in an automatic transmission form. Thus, significant increases in fuel economy and vehicle performance may be achieved through the effective use of certain automated manual transmissions.
The dual clutch transmission structure may include two dry disc clutches each with their own clutch actuator to control the engagement and disengagement of the two-clutch discs independently. While the clutch actuators may be of the electromechanical type, since a lubrication system within the transmission requires a pump, some dual clutch transmissions utilize hydraulic shifting and clutch control. These pumps are most often gerotor types, and are much smaller than those used in conventional automatic transmissions because they typically do not have to supply a torque converter. Thus, any parasitic losses are kept small. Shifts are accomplished by engaging the desired gear prior to a shift event and subsequently engaging the corresponding clutch. With two clutches and two inputs shafts, at certain times, the dual clutch transmission may be in two different gear ratios at once, but only one clutch will be engaged and transmitting power at any given moment. To shift to the next higher gear, first the desired gears on the input shaft of the non-driven clutch assembly are engaged, then the driven clutch is released and the non-driven clutch is engaged.
This requires that the dual clutch transmission be configured to have the forward gear ratios alternatingly arranged on their respective input shafts. In other words, to perform up-shifts from first to second gear, the first and second gears must be on different input shafts. Therefore, the odd gears will be associated with one input shaft and the even gears will be associated with the other input shaft. In view of this convention, the input shafts are generally referred to as the odd and even shafts. Typically, the input shafts transfer the applied torque to a single counter shaft, which includes mating gears to the input shaft gears. The mating gears of the counter shaft are in constant mesh with the gears on the input shafts. The counter shaft also includes an output gear that is meshingly engaged to a gear on the output shaft. Thus, the input torque from the engine is transferred from one of the clutches to an input shaft, through a gear set to the counter shaft and from the counter shaft to the output shaft.
Gear engagement in a dual clutch transmission is similar to that in a conventional manual transmission. One of the gears in each of the gear sets is disposed on its respective shaft in such a manner so that it can freewheel about the shaft. A synchronizer is also disposed on the shaft next to the freewheeling gear so that the synchronizer can selectively engage the gear to the shaft. To automate the transmission, the mechanical selection of each of the gear sets is typically performed by some type of actuator that moves the synchronizers. A reverse gear set includes a gear on one of the input shafts, a gear on the counter shaft, and an intermediate gear mounted on a separate counter shaft meshingly disposed between the two so that reverse movement of the output shaft may be achieved.
While these power-shift dual clutch transmissions have overcome several drawbacks associated with conventional transmissions and the newer automated manual transmissions, it has been found that controlling and regulating the automatically actuated dual clutch transmission to achieve the desired vehicle occupant comfort goals is a complicated matter. There are a large number of events to properly time and execute within the transmission for each shift to occur smoothly and efficiently. In addition, the clutch and complex gear mechanisms, working within the close confines of the dual clutch transmission case, generate a considerable amount of heat. The heat build-up is aggravated by the nature of the clutch mechanisms themselves, each of which are typically constructed of two series of plates, or discs, one set connected in some manner to the output of the engine and the second attached to an input shaft of the transmission. Each of the set of plates include friction material. The clutch plates and discs are pressed together under pressure to a point at which the plates and discs make a direct physical connection. The clutch may be designed for a full “lock-up” of the plates and discs, or may be designed with a certain amount of “limited slip”. Regardless, the slipping of the friction plates within a friction type clutch, whether from a designed limited slip or the normal uncontrolled slipping that occurs during clutch engagement and disengagement, generates heat that needs to be dissipated. A considerable amount of heat can be generated in the typical dual clutch transmission utilizing a combined coaxial clutch assembly wherein the one clutch fits within the second clutch.
In order to provide sufficient cooling to the clutch assemblies of the conventional dual clutch transmission, the clutch assemblies are usually bathed in transmission fluid in a generally uncontrolled manner. While this approach has generally worked for its intended purpose, disadvantages remain. Specifically, these types of conventional clutch cooling hydraulic circuits have failed either to adequately provide for proper cooling and heat reduction of the clutches of the dual clutch transmission or have resulted in producing large efficiency losses by excessively flooding of the clutch assemblies with fluid.
Of late, newer approaches in the structure of hydraulic circuits for clutch cooling have been proposed in the related art that offer improvements, but are still limited in their cooling capacity. In one example, a hydraulic circuit is employed that provides pressurized cooling oil directly from the pump to the clutches to maintain adequate flow and pressure and then passes the cooling fluid through a cooler device only on the return line to the sump. However, since the cooling fluid is not cooled on its way to the clutches, the cooling capacity of the applied cooling fluid is somewhat limited and larger quantities of fluid must be employed to adequately cool the clutch. This generally requires a larger pump and larger supply lines. In another example, the pressurized cooling fluid is pumped through a cooling unit prior to delivery to the clutches. However, this causes a distinct pressure drop and the system must contend with the flow limitations of the cooling unit.
These conventional clutch cooling approaches also use a single hydraulic circuit to supply cooling oil or fluid from the cooler device to the clutches. This causes the clutches to suffer inadequate and inefficient heat removal. Furthermore, the inadequacy of these conventional hydraulic circuits is also exaggerated under clutch high loading conditions where excessively high heat is built up rapidly in the active clutch. These inherently inadequate cooling circuit strategies lead to shortened component life and ultimate failure of the clutch assemblies within the dual clutch transmission. Similarly, inadequate cooling is responsible for rapid breakdown of the physical properties of the transmission fluid, which can cause failure of the other components within the transmission. Further, the conventional hydraulic circuits that excessively flood the clutch assemblies with cooling fluid also cause unnecessary clutch drag and put excessive demands on the pump resulting in poor clutch life and lower fuel efficiencies.
Accordingly, there remains a need in the related art for an improved hydraulic circuit to provide cooling fluid to the clutch assemblies of the dual clutch transmissions. Specifically, there is a need for a cooling circuit that provides cooling fluid from a cooling unit to the clutches normally and supplementally provides high flow cooling fluid from the pump under high load conditions. In this manner, clutch heat is dissipated by cooled cooling fluid normally and a supplemental flow of cooling fluid that is not limited by the flow restrictions of the cooling unit is further provided when the clutch comes under high load.